The present invention relates generally to apparatus and methods for maintaining patency in body passages and more particularly to a catheter system capable of selectively ablating occlusive media within a body lumen. The present invention is particularly useful for the electrosurgical cutting or ablation of invasive tissue growth in and around a stent anchored in the body lumen to help reduce or eliminate restenosis of the body lumen.
When a patient is suffering from atherosclerosis, significant occlusions or blockages are formed on the interior wall of the artery. As a result of these occlusions, the organ or extremity to which blood is to be supplied is compromised and the patient may experience a myocardial infarction or stroke. In less severe cases, it is often sufficient to treat the symptoms with pharmaceuticals and lifestyle modification to lessen the underlying causes of the disease. In more severe cases, a coronary artery blockage can often be treated using endovascular techniques such as balloon angioplasty, atherectomy, laser or hot tip ablation, placement of stents, and the like.
Percutaneous transluminal balloon angioplasty (PTBA) has become a recognized method of reducing the occlusion of blood vessels. The procedure involves routing a catheter having an inflatable balloon at the distal end thereof through the vascular system until the balloon is positioned at the site of the stenotic lesion to be treated. The balloon is then inflated to compress the atherosclerotic plaque into the wall of the blood vessel, thus increasing the size of the opening and enhancing blood flow through the affected artery. However, this successful procedure is overshadowed by the occurrence of restenosis, a re-narrowing of the artery. Studies have shown that 30-40 percent of angioplasty patients experience restenosis within 3-6 months of the angioplasty procedure. When restenosis occurs, patients may be treated with cardiovascular medications, additional angioplasty procedures or bypass surgery.
Restenosis often occurs because the wall of the dilated artery tends to spring back to its original shape following deflation of the dilation balloon. Arterial stenting has been introduced as a solution to the recoil of the vessel wall. Arterial stenting involves the placement of an expandable coil spring or wire-mesh tube within the occluded artery to reopen the lumen of the blood vessel. One example of an arterial stent is disclosed in U.S. Pat. No. 4,739,792 to Julio Palmaz. The Palmaz device comprises an expandable wire-mesh graft or prosthesis which is mounted upon an inflatable balloon catheter. The catheter assembly, including the graft, is delivered to the occluded area and is then inflated to radially force the graft into contact with the occlusion. As the graft expands, the lumen of the blood vessel is opened and blood flow is restored. After complete expansion of the graft, the balloon catheter is deflated and removed, leaving behind the graft to buttress and prevent elastic recoil of the blood vessel wall.
Although this method is successful in preventing recoil of the vessel wall, restenosis will often still occur. Smooth muscle cells which form the vessel wall tend to proliferate and build-up in the newly stented area of the blood vessel. This cellular buildup may eventually become large enough to block the lumen of the blood vessel.
It has recently been determined that localized heating of the blood vessel wall may inhibit the proliferation of smooth muscle cells which are believed to cause restenosis. One example of localized blood vessel heating is disclosed in U.S. Pat. No. 4,799,479 to Spears. The Spears patent discloses an apparatus for angioplasty having an inflatable balloon catheter which is provided with a meshwork of electrical wires to supply heat to a vessel wall. Following balloon angioplasty, the external surface of the balloon is heated to fuse together disrupted tissue elements and to kill smooth muscle cells which are believed to lead to restenosis. Unfortunately, the Spears device does not adequately prevent the spontaneous elastic recoil of the arterial wall. Immediately following angioplasty, the arterial wall begins to spring back to its original shape.
Thus stenting in and of itself is ineffective in preventing restenosis due to the occurrence of cellular proliferation. Likewise, balloon dilation in combination with localized heating does not adequately prevent restenosis since the vessel wall tends to spontaneously return to its original occluded shape.
Other techniques have recently been developed to help reduce incidences of restenosis. For example, procedures for irradiating the angioplasty site with UV light to reduce the proliferation of smooth muscle cells at the site have been disclosed. In addition, techniques have been disclosed for the controlled application of thermal and/or electrical energy directly to the stent by, for example, including resistive or inductive heating elements that may include radiofrequency electrodes within the stent. The radiofrequency energy is then applied to the stent to disrupt the cellular growth in or around the stent. One major disadvantage of these procedures is that it is difficult to selectively apply the energy to the invasive tissue without causing thermal damage to the body lumen wall. In particular, methods that apply energy, such as RF energy, directly to the stent will often cause thermal damage to the surrounding body lumen in which the stent is anchored.
The present invention comprises apparatus and methods for maintaining patency in body passages subject to occlusion by invasive tissue growth. The apparatus and methods of the present invention may be used to open and maintain patency in virtually any hollow body passage which may be subject to occlusion by invasive cellular growth or invasive solid tumor growth. Suitable hollow body passages include ducts, orifices, lumens, and the like, with exemplary body passages including the coronary arteries. The present invention is particularly useful for reducing or eliminating the effects of restenosis in coronary arteries by selectively removing tissue ingrowth in or around stents anchored therein.
The principles of the present invention are generally applicable to any body lumen which becomes partially or totally occluded. Methods of the present invention comprise advancing an electrosurgical catheter within the body passage such that an electrode terminal is positioned near the occlusive media. High frequency voltage is applied to one or more electrode terminal(s) at the distal end of the catheter such that an electrical current flows from the electrode terminal(s), through the region of the occlusive media, and to a return electrode to volumetrically remove the occlusive media in situ. In exemplary embodiments, the high frequency voltage is sufficient to effect molecular dissociation or disintegration of the occlusive media, thus converting the solid media into non-condensable gases.
The present invention is particularly useful in a lumen containing a lumenal prosthesis, such as a stent, stent-graft or graft, which may be metallic, non-metallic or a non-metallic coated metallic structure. Restenosis often occurs when arthermateous media or thrombus moves or grows through or around the cylindrical wall of the prosthesis to partially occlude the body passage. Methods of the present invention comprise advancing an electrosurgical catheter within the body passage such that an electrode terminal is positioned near the occlusive media. High frequency voltage is applied to one or more electrode terminal(s) at the distal end of the catheter such that an electrical current flows from the electrode terminal(s), through the region of the occlusive media, and to a return electrode to selectively remove the occlusive media without directly applying thermal or electrical energy to the prosthesis or the lumenal wall. The electrode terminal may then be advanced through the vacancy left by the removed occlusive media to recanalize the vessel. By selectively removing the occlusive media without passing energy directly to the stent, thermal damage to the surrounding lumenal wall is minimized.
In an exemplary embodiment, the return electrode is located on the catheter so that the current flow paths are confined between the return electrode and one or more electrode terminals in the vicinity of the working end of the catheter. This confinement of current flow paths minimizes the undesired flow of current through portions or all of the stent, which may otherwise induce non-specific tissue injury beyond the site of recanalization of the occluded lumen. In one configuration, the return electrode is a movable guide wire positioned radially inward from the electrode terminal such that the electrical current flows from the electrode terminal radially inward to the return electrode, thereby inhibiting current flow through the prosthesis. In another embodiment, the return electrode is an annular band positioned proximal of the electrode terminal(s).
In preferred embodiments, the high frequency voltage is applied in the presence of electrically conducting fluid such that a current flow path is generated between the electrode terminal(s) and the return electrode through the electrically conducting fluid. Preferably, the electrically conductive fluid is delivered through an internal lumen in the catheter (or through a separate instrument) to a region around the occlusive media to displace naturally occurring bodily fluids. This region may be fluidly isolated to confine the electrically conducting fluid around the tissue ablation site. In one embodiment, the region is isolated by advancing proximal and distal balloons to either side of the region, and inflating these balloons to effect a seal with the interior wall of the body passage.
Once the target site is isolated from the rest of the vasculature, the supply of electrically conductive fluid is continuously delivered to the region and balanced with the aspiration of fluid from the site of intended recanalization. The electrode terminal(s) are energized by applying a high frequency voltage between electrode terminal(s) and the return electrode, which can be a movable guide wire. A high electric field is created at the surface of the electrode(s) which causes the volumetric removal or ablation of target tissue in close proximity with the electrode terminal(s). As the occlusive media is ablated, gaseous products are generated which are entrained in the electrically conducting fluid and removed through the aspiration lumen in the catheter. The current flux lines are generally confined to the central portion of tissue ablation region because they generally flow inward towards the return electrode and because the occlusive media generally shields the outer region of the body passage (including the stent) from the current flux lines. This minimizes undesirable interaction between the electrical current and the stent. In an exemplary embodiment, the distal portion of the catheter body is reciprocally rotated as the electrode terminal is energized to selectively ablate the occlusive media. The catheter body is then advanced through the vacancy left by the ablated occlusive media to recanalize the vessel.
In a specific configuration, the occlusive media is removed by molecular dissociation or disintegration processes. In these embodiments, the high frequency voltage applied to the electrode terminal(s) is sufficient to vaporize an electrically conductive fluid (e.g., saline or blood) between the electrode terminal(s) and the occlusive media. Within the vaporized fluid, a ionized plasma is formed and charged particles (e.g., electrons) are accelerated towards the target media to cause the molecular breakdown or disintegration of several cell layers of the media. This molecular dissociation is accompanied by the volumetric removal of the media. The short range of the accelerated charged particles within the plasma layer confines the molecular dissociation process to the surface layer to minimize damage and necrosis to the surrounding blood vessel walls. This process can be precisely controlled to effect the volumetric removal of tissue or media as thin as 10 to 150 microns with minimal heating of, or damage to, surrounding or underlying tissue structures. A more complete description of this phenomena is described in commonly assigned U.S. Pat. No. 5,683,366, the complete disclosure of which is incorporated herein by reference.
Apparatus of the present invention comprise a catheter shaft having a flexible body with a proximal end portion and a distal end portion with one or more electrode terminal(s), and a connector extending through the body for coupling the electrode terminal(s) to a source of high frequency voltage. Upon the application of sufficient high frequency voltage to the electrode terminal(s), the occlusive media is volumetrically removed from the body lumen to recanalize the body lumen. In some embodiments, the apparatus will further include one or more fluid delivery element(s) for delivering electrically conducting fluid to the electrode terminal(s) and the target site. The fluid delivery element(s) may be located on the catheter, e.g., one or more fluid lumen(s) or tube(s), or they may be part of a separate instrument. In an exemplary embodiment, the electrically conducting fluid will preferably generate a current flow path between the electrode terminal(s) and one or more return electrode(s). In an exemplary embodiment, the return electrode(s) are located on the catheter and spaced a sufficient distance from the electrode terminal(s) to substantially avoid or minimize current shorting therebetween and to shield the return electrode(s) from tissue at the target site. Alternatively, the return electrode(s) may comprise a dispersive pad located on the outer surface of the patient (i.e., a monopolar modality).
In a specific configuration, the apparatus includes a plurality of electrically isolated electrode terminals extending from the distal end of the catheter shaft. The electrode terminals are each mounted within an electrically insulating support member, and spaced peripherally around the distal opening of the catheter body. In these embodiments, the catheter may include a single, annular return electrode located proximal of the distal opening, or a plurality of electrode terminals mounted to the support members proximal of the electrode terminals. In this embodiment, the catheter body also includes one or more fluid delivery lumens spaced peripherally around the central lumen for delivering electrically conductive fluid to the electrode terminals. In addition, the catheter body will preferably include one or more suction lumens spaced peripherally around the central lumen, and suitably coupled to an external suction source for aspirating fluid, tissue and/or gaseous products of ablation (e.g., non-condensable gases) from the target site.
In an exemplary embodiment, the working end portion of the catheter has an adjustable outer diameter to facilitate advancement of this portion of the catheter through a variable diameter body lumen or stent. In one configuration, the working end of the catheter will taper in the distal direction (e.g., in a series of steps) so that the surgeon can advance the catheter through a severely occluded body lumen. The catheter may include a series of axially spaced electrode terminal(s) that are electrically isolated from each other to allow for each set to be independently activated. By way of example, in a severely occluded body lumen, the surgeon may activate the distal set of electrode terminal(s) to remove the innermost occlusive media, advance these distal electrode terminal(s) through the vacancy left by the removed occlusive media, and then activate a more proximal, and radially outward, set of electrode terminal(s) to remove occlusive media radially outward from the initially removed media.
In another configuration, the working end of the catheter may be radially expandable and compressible so that the diameter of the working end can be varied as the catheter is advanced through the lumen. In some instances, stents will not expand uniformly resulting in portions of the stent having smaller inner diameters. In other instances, vessel wall pressure may cause portions of the stent to spring back to its original shape or partially back to this shape so that the overall inner diameter of the stent varies in the axial direction. Accordingly, the present invention allows the diameter of the working end of the catheter to vary (either automatically in response to the body lumen or stent inner diameter, or through activation by the surgical team) to facilitate advancement through non-uniform stents or body lumens.
In another embodiment of the invention, the catheter system includes a high frequency power supply configured to reduce or interrupt power when the electrode terminal(s) contact a low impedance object, such as a stent within the body lumen. In one embodiment, the power supply includes a spark prevention device for eliminating or reducing sudden pulses in current when an instrument powered by the power supply contacts a low impedance source. The spark limiting device is coupled to one or more current sensors on the electrode terminal(s) to substantially continuously monitor current output, interrupting current output from the output driver when current output from the output current sensor exceeds a predetermined threshold level. The spark prevention mechanism, which may be used in conjunction with other power limiting devices, preferably turns off output from the power supply when output current from the supply exceeds a predetermined current level.